206 AMPHIMIXIS OR ESSENTIAL MEANING OF [XII. 



ovum. The latter also dies for a similar reason when it has not 

 been fertilized. Some years ago I described the different manner 

 in which the eggs of two closely allied species of Crustacea 

 behave when they have no prospect of being fertilized ^ If a 

 female of Moma paradoxa, bearing winter-eggs in the ovary, be 

 separated from the males, it nevertheless deposits its ova in the 

 brood-chamber, but they utterly disintegrate in a few hours and 

 are washed away by the water as it flows through the chamber. 

 It is very different with Moina 7'ectirostris ; the winter-egg, when 

 ripe and ready to pass into the brood-chamber, almost occupies 

 the entire ovary. When males are absent and fertilization does 

 not occur, the ^gg is not laid but is retained by the isolated 

 female in her ovary in which it remains apparently un- 

 changed for many days, probably quite capable of being fer- 

 tilized. Finally it changes in appearance, losing its uniform 

 finely granular look, while the fat-globules and particles 

 of albumen fuse together into great irregular masses which 

 are presentl}'- rather rapidly reabsorbed. Instead of winter- 

 eggs the parthenogenetic summer-eggs are now formed, and 

 we may maintain that the material of the former is not lost 

 to the individual or to the species when fertilization is excluded, 

 but is converted into new ova which do not require fertilization. 

 No one can doubt that the habit of laying the winter-egg only 

 after the stimulus provided by fertilization, is an adaptation ; but 

 who would explain in this manner the destruction of the un- 

 fertilized egg, which remains in the ovary t This destruction is 

 certainly not purposeless ; but there are cases of unintended 

 usefulness, and other species of Moina prove that this is one of 

 them, for the unfertilized eggs are destroyed in the brood- 

 chamber (where their material is lost). The destruction is 

 therefore no adaptation but merely a consequence of the con- 

 stitution of the egg which is so altered by preparation for the 

 fertilization which should have ensued, that it can neither de- 

 velope into an embryo nor continue to live. It is just the same, 

 if I mistake not, with Infusoria ; the gradual destruction of 

 those animals which do not conjugate is no special adaptation, 

 but rather an inevitable consequence of the necessary internal 



' Weismann, * Beitrage zur Naturgeschichte der Daphnoiden,' Leipzig, 

 1876-79. Abhandlung IV. ' Ueber den Einfluss der Begattung auf die 

 Erzeugung von Wintereiern.' i 



